ANALYSIS OF LOCAL ORDER IN THE SPATIAL-DISTRIBUTION OF CELL-SURFACE MOLECULAR ASSEMBLIES

Electron micrographs obtained from platinum-carbon replicas of freeze-cleaved or freeze-dried cells and exhibiting intramembranous or externally-disposed particles were analysed statistically for degree of non-random particle association. Analysis was based on use of an analogy to the radial distribution function, g(r), which provided the average density of neighbor particles around a particle in the micrograph at 30 Å increments from particle center. The ratio of average density of neighbors around a particle to overall particle density in the micrograph was then determined as a function of distance from particle center. For a random plot of 90 Å particles, the computed ratio deviated from 1 by less than 10% for a distance up to 900 Å from particle center. In a field of similarly-sized intramembranous particles within the freeze-cleaved surface membrane of a mouse fibroblast, there were indications of slight non-random particle associations at distances of 120-240 Å and 300-330 Å from particle center, while a significant association of particles comprising an intercellular gap junction was obtained at 90-120 Å. Virus-related molecular assemblies on the surfaces of fibroblasts infected with a temperature-sensitive mutant of murine luekemia virus and grown under conditions that did not permit virus assembly to occur were analysed similarly. Significant deviation from randomness was found at several distances from particle center, both on areas of membrane that were populated by large numbers of particles and on low-density areas. © 1979.